1. Field of the Invention
The present invention relates to a copper conductor paste and a production method of a copper conductor film, and more specifically to a copper conductor paste forming conductive paths of a copper film by printing the copper paste onto a ceramic substrate followed by burning and also to a production method of a copper conductor film.
2. Description of the Related Art
Recently, as a conductive paste being applied to electric and electronic parts such as ceramic substrate hybrid IC, ceramic condensers, etc., an Ag-Pd paste mainly composed of silver and palladium, a gold paste, an Ag-Pt paste mainly composed of silver had platinum, and a copper paste are used.
In these pastes, the Ag-Pd paste is a typical paste as a wiring use but has some disadvantages. For example, when the paste is used as wiring on a substrate, silver is ionized with moisture in air. Consequently, a phenomenon called a migration is caused in which the ionized silver migrates to the adjacent conductor paths to cause short-circuiting. Thus, the distance between the conductor paths cannot be narrowed. Also, in a soldering portion for loading or connecting other part onto a conductor path, silver is liable to be corroded with a soft solder and the Ag-Pd paste is inferior in the soldering resistance.
Also, in the case of adhering the foregoing Ag-Pd paste to a substrate, since fine metal particles of a micron size cannot essentially be adhered to a ceramic substrate by causing a reaction, about 4 to 10% by weight a glass frit is compounded with the paste and the glass frit existing on the substrate after printing fives a function of adhering the substrate and the metal film after burning. However, on the other hand, since a large amount of the glass frit remains in the metal film after burning, there occur the problems that the electric resistance of the metal film is increased. Also, since the metal film is adhered to the substrate with a glass layer, a strain is liable to occur by the difference in the thermal expansion and the thermal shock resistance is weakened.
As a paste partially solving such disadvantages, a copper paste is known. The paste is composed of a composition formed by dispersing copper, a glass frit, and a non-copper material such as tungsten, molybdenum, rhenium, etc., in an organic solvent as described, e.g., in Unexamined Japanese Patent Publication (kokai) No. Sho. 60-70746. Also the paste is composed of a composition formed by dispersing metallic copper particles coated with copper oxide, copper oxide particles, and a glass powder in an organic solvent as described in Examined Japanese Patent Publication (kokoku) No. Hei. 3-50365.
However, since in the foregoing copper paste, a large amount of, preferably from 5 to 10% by weight, the glass frit is added as a glass powder and functions for adhering the substrate and the metal film, when the copper paste is coated on a substrate and a metal film is adhered to the substrate by burning, a large amount of the glass frit remains in the metal film after burning and thus there still remain problems that the electric resistance of the metal film is high, also the glass layer existing at the interface between the metal film and the substrate causes a strain by the difference in the thermal expansion, and the heat resistance and the thermal shock resistance are weakened. The thermal shock resistance is evaluated by the adhesive force between the metal film and the substrate after transferring the substrate having adhered thereto the metal film from a low-temperature atmosphere to a high-temperature atmosphere and repeatedly transferring the substrate to the opposite direction.
Also, since a lead borosilicate glass having a low softening point is used as the foregoing glass frit, in the soldering step for an oxidation prevention and Au wire bonding, lead in the foregoing glass inhibits soldering.